Source for 4.2mH inductor?

[billbillw]

Looks like I need to replace a pair of Iron core inductors in the low-pass section of the crossover on my Mirage M3 speakers.
I picked them up last fall and noticed after awhile that the frequency response out of each woofer is different. One woofer passes much more midrange. Its a basic 2nd order circuit, and bi-wireable, so I check the woofers on their own. The original schematic shows a 4.2mH inductor, 0.6ohm. When I look at it, it appears to be an iron core inductor. I've already replaced all the shunt caps, so its not the caps, pretty much gotta be the inductor coils. (I don't have an inductance meter to test them).

Anyway, I need to find a source for a 4.2 mH iron core inductor. Can't seem to find that exact value at any of the places I've looked (Partsexpress, Madisound, Partsconnection.
Any ideas?

[Cloth Ears]

I know that E-speakers used to have Mundorf, and they have a huge range of both air- and ferrite-cored inductors, although they don't have any 4.2mH. The closest would be any 3.9 in series with a 0.33 (and be careful of alignment.

You could probably find pairs with DCR less than .5ohm (depending on your budget).

Other places in the US might have Mundorf, so do a search (they're not all expensive).

[Apex Jr]

I have some 4.75mH Iron core chokes in stock

There $1.50 each

Steve @ Apex Jr.

[billbillw]

I'd rather stay away from multiple inductors. The space on the x-over board is very tight, and there's not much room inside the cabinet to separate them. I'd be afraid of a mutual inductance affecting the values.

I might just have to buy a slightly oversized inductor and remove a few windings until I get the right value. Of course, that would mean that I'd have to finally buy an inductance meter.

Any other suggestions?

[I_Forgot]

Have you checked the resistance of the inductors? Inductors rarely fail open unless someone's amp died and put the PS voltage on the output and literally melted a wire. You don't need to measure inductance- it doesn't change. Just measure resistance with a meter. If it reads close to 0 Ohms the inductor is fine. If it reads open, you have found your problem.

I_F

[billbillw]

Originally posted by I_Forgot
Have you checked the resistance of the inductors? Inductors rarely fail open unless someone's amp died and put the PS voltage on the output and literally melted a wire. You don't need to measure inductance- it doesn't change. Just measure resistance with a meter. If it reads close to 0 Ohms the inductor is fine. If it reads open, you have found your problem.

I_F

Correct me if I'm wrong, but if the coil were open, there would be no sound. The inductor is wired in series with the speaker. I don't believe the wire has to melt, it just has to get hot enough for the varnish to burn off and let the coil wires make contact, which would change the length of coil in the inductor, thus changing the value of inductance. I didn't think it was possible, but given the history of these speakers, and other observed damage on the circuit boards, I do believe that an amp did die at one point and the speakers saw large amounts of DC current. Some of the cement encased resistors got so hot, they discolored the board. At least one poly cap blew up too. I wouldn't be surprised if all the drivers had to be replaced at some point. I know for sure the woofers were replaced not long before I obtained them.

[Cloth Ears]

Originally posted by billbillw
Looks like I need to replace a pair of Iron core inductors in the low-pass section of the crossover on my Mirage M3 speakers.
I picked them up last fall and noticed after awhile that the frequency response out of each woofer is different. One woofer passes much more midrange. Its a basic 2nd order circuit, and bi-wireable, so I check the woofers on their own. The original schematic shows a 4.2mH inductor, 0.6ohm. When I look at it, it appears to be an iron core inductor. I've already replaced all the shunt caps, so its not the caps, pretty much gotta be the inductor coils. (I don't have an inductance meter to test them).

Anyway, I need to find a source for a 4.2 mH iron core inductor. Can't seem to find that exact value at any of the places I've looked (Partsexpress, Madisound, Partsconnection.
Any ideas?

billbillw,

I'd be doing a bit of checking before I started looking for new inductors. It's very inlikey that they have become damaged enough to change the sound like you mention (and still be working). Have you tried 'simply' swapping the inductors between the 2 crossovers and seeing if the 'problem' swaps along with the inductors? My guess is that it will not and you'll find the problem in either your drivers or the caps in one or other crossover.

Do you have a copy of the original schematic for us to look at?

[billbillw]

Originally posted by Cloth Ears


billbillw,

I'd be doing a bit of checking before I started looking for new inductors. It's very inlikey that they have become damaged enough to change the sound like you mention (and still be working). Have you tried 'simply' swapping the inductors between the 2 crossovers and seeing if the 'problem' swaps along with the inductors? My guess is that it will not and you'll find the problem in either your drivers or the caps in one or other crossover.

Do you have a copy of the original schematic for us to look at?

I thought the same thing, but its gotta be true. I haven't swapped the inductors, but I've swapped the whole crossover from one side to the other, so its not the speaker or internal cabinet wiring. The problem follows the x-over. The low pass section of the crossover is independent from the rest, and fairly simple, just the coil and 3 shunt caps, all of which I just replaced, so by my logic, its gotta be the coil(s).

Here is the schematic:

[I_Forgot]

In that case, how about just rewinding the coils? Take one apart, counting turns as you unwind it. Put the same number of turns of fresh wire back on the core.

I_F

[billbillw]

I could try that, but I'd probably feel better if I picked up an LCR meter to test my work out.